Current vehicle navigation systems use GPS, such as an electromagnetic wave positioning system, to determine a vehicle's position. Such a GPS system is the Navigation Satellite Timing and Ranging (NAVSTAR) Global Positioning System, which is a space-based satellite radio navigation system developed by the U.S. Department of Defense (DoD). GPS includes NAVSTAR GPS and its successors, Differential GPS (DGPS), or any other electromagnetic wave positioning systems. NAVSTAR GPS receivers provide users with continuous three-dimensional position, velocity, and time data.
NAVSTAR GPS consists of three major segments: Space, Control, and User as illustrated in FIG. 1. The space segment 2 consists of a nominal constellation of 24 operational satellites which have been placed in 6 orbital planes above the Earth's surface. The satellites are in circular orbits in an orientation which normally provides a GPS user with a minimum of five satellites in view from any point on Earth at any one time. The satellites broadcast an RF signal which is modulated by a precise ranging signal and a coarse acquisition code ranging signal to provide navigation data.
This navigation data, which is computed and controlled by the GPS control segment 4, includes the satellite's time, its clock correction and ephemeris parameters, almanacs, and health status for all GPS satellites. From this information, the user computes the satellite's precise position and clock offset.
The control segment consists of a Master Control Station and a number of monitor stations at various locations around the world. Each monitor station tracks all the GPS satellites in view and passes the signal measurement data back to the master control station. There, computations are performed to determine precise satellite ephemeris and satellite clock errors. The master control station generates the upload of user navigation data from each satellite. This data is subsequently rebroadcast by the satellite as part of its navigation data message.
The user segment 6 is the collection of all GPS receivers and their application support equipment such as antennas and processors. This equipment allows users to receive, decode, and process the information necessary to obtain accurate position, velocity and timing measurements. This data is used by the receiver's support equipment for specific application requirements. GPS supports a wide variety of applications including navigation, surveying, and time transfer.
GPS receivers may be used in a standalone mode or integrated with other systems. Currently, land-based navigation systems use vehicle speed sensor, rate gyro and a reverse gear hookup to "dead reckon" the vehicle position from a previously known position. This method of dead reckoning, however, is susceptible to sensor error, and therefore requires more expensive sensors for accuracy and dependability.
The systems that use odometers, gyros and reverse gear hookups also lack portability due to the required connections to odometers and the frailty of gyros. Moreover, these systems are hard to install in different cars due to differing odometer configurations which can have different connections and pulse counts in the transmission. Odometer data also varies with temp, load weight, tire pressure, speed. Alternative connections to cruise control or ABS sensors bring up safety concerns.
Prior systems use a road network stored in a map database to calculate current vehicle positions. These systems send distance and heading information to perform map matching, and map matching calculates the current position based on the road network and the inputted data. These systems also use map matching to calibrate sensors. Map matching, however, has inherent inaccuracies because map matching must look back in time and match data to a location. As such, map matching can only calibrate the sensors when an absolute position is identified on the map, but on a long, straight stretch of highway, sensor calibration using map matching may not occur for a significant period of time.
Accordingly, there is a need for a potentially portable, vehicle navigation system which is flexible, accurate, efficient and cost-effective in determining a current position from a previous position.